Fix kill command syntax in set_wallpaper script (varietywalls#802)

* Fix kill command syntax in set_wallpaper script

`kill` was failing in cases when multiple `swaybg` instances were running simultaneously, because it was receiving a string of PIDs as a single argument. Removing the double quotes makes it effectively end any and all `swaybg` processes. Running processes were accumulating due to this fault, making this a chinken-and-egg problem. Solved, anyway.

* Loop on multiple swaybg processes

Refactor PID handling to support multiple processes using a loop, in order to comply to shellcheck SC2046.

* Fix syntax errors in set_wallpaper script

Author: fde-capu

data/scripts/set_wallpaper

@@ -245,7 +245,10 @@ elif [ "$DE" == "sway" ]; then
         swaybg -i "$WP" -m fill &
         if [ -n "$PID" ]; then
             sleep 1
-            kill "$PID" 2>/dev/null
+            read -r -a PID <<< "$PID"
+            for each_pid in "${PID[@]}"; do
+                kill "$each_pid" 2>/dev/null
+            done
         fi
     else
         swaymsg output "*" bg "$WP" fill 2>/dev/null